The optics and electronics industries rely upon inorganic compounds for fabrication of various components. However, these industries may benefit largely from the plethora of organic compounds, both known and unknown. Of the many potential applications of organic compounds to the above-mentioned industries, many relate to the electro-optic effect as described by Kerr (1875) and Pockels (1906). Additionally, frequency doubling by second harmonic generation (SHG) is often considered. SHG may be defined as the doubling of light's fundamental frequency.
A test to study SHG has been developed (Kurtz and Perry, 1968) which analyzes, for instance, the noncentrosymmetric crystal structure of organic compounds. Organic compounds which posses a noncentrosymmetric structure exhibit optical nonlinearity and are generally said to be nonlinear.
Organic nonlinear optical materials displaying high SHG properties are potentially useful in applications which require high speed optical modulators. Such applications include high speed long distance data links and electric field sensors for use in electromagnetically noisy environments. In addition, such materials provide efficient wavelength shifting capability for optical and infrared remote sensing (e.g., of pollutant particulate concentration) and diode laser frequency doubling for optical data storage.
It has been of increasing interest to prepare organic nonlinear optical materials, such as stilbazolium salts, that posses high melting points without adversely affecting their SHG properties. High melting point stilbazolium salts are desirable because many processing steps involving such materials are conducted at temperatures that are near or greater than their conventional melting points. This often causes molecular breakdown or molecular restructuring of the materials which inevitably results in loss of nonlinear optical properties.
The present invention, therefore, is based on the discovery of stilbazolium salts that display high SHG properties as well as high melting points. High SHG properties may be defined as a second harmonic generation powder efficiency of at least about 1000 as compared to a urea standard which is assigned a value of 1 (SHG powder efficiency as described by Marder et al., Science, 245, 626-628 (1989). High melting points may be defined as at least about 230.degree. C.
Description of the Prior Art
Accordingly, attempts have been made to prepare stilbazolium salts with high melting points. In commonly assigned U.S. Pat. No. 5,094,553, 4'-dimethylamino-4-methylstilbazolium p-toluenesulfonate (DAST) is disclosed. Said DAST has a favorable melting point range. However, as a result of its toluenesulfonate anion, the molecular dipoles within the crystals of the compound form a herringbone arrangement which characteristically reduces their SHG properties.
Other investigators have focused their attention on stilbazolium salts that posses favorable SHG properties. In commonly assigned U.S. Pat. No. 5,194,984, 4'-dimethylamino-4-methylstilbazolium methanesulfonate (DASMS) is disclosed. Crystals of said DASMS possess favorable SHG properties. However, since DASMS crystals contain a tetrahydrated methanesulfonate anion, they melt at lower temperatures and are more difficult to process.
Efforts to produce stilbazolium salts that unexpectedly retain high SHG properties and high melting points have not been disclosed.
The instant invention, therefore, is patentably distinguishable from the above-mentioned patents, since among other reasons, it is based on the discovery of stilbazolium salts that unexpectedly display high second harmonic generation properties as well as high melting points.